1. Field of Invention
This invention concerns the manufacture of 3-isothiazolones, and in particular it is concerned with a process for preparing 3-isothiazolones containing little or no salt ("salt-free"), and also little or no water ("water-free").
2. Description of the Prior Art
3-Isothiazolones of the formula: ##STR3## wherein Y is an alkyl or substituted alkyl of 1 to 10 carbon atoms; an unsubstituted or halogen substituted alkenyl or alkynyl of 2 to 10 carbon atoms; an aralkyl or halogen-, lower alkyl-, or lower alkoxy-substituted aralkyl of up to 10 carbon atoms; X is hydrogen or a (C.sub.1 -C.sub.2)alkyl; and X.sup.1 is hydrogen, chlorine, or a (C.sub.1 -C.sub.2)alkyl are disclosed in U.S. Pat. Nos. 3,523,121 and 3,761,488.
These 3-isothiazolones are well known as microbicides and are employed in many industrial and household systems. Since the 3-isothiazolones in aqueous solutions are generally unstable, stabilizing divalent metal salts as described in U.S. Pat. Nos. 3,870,795 and 4,067,878 are usually incorporated.
In certain applications, e.g., preservation of latex emulsion, these metal stabilization salts cause problems which can reduce the performance or value of such systems.
Another problem with such metal stabilization salts is that they cause corrosion in certain systems. For example, chloride salts have a corrosive effect on many metals and are to be avoided where possible. In water treatment systems where low cation and anion levels are important, it is desirable to eliminate such salts. In the stabilization of plastic articles, salts may contribute to deterioration of optical properties and/or increase water pickup and haze levels.
In some cosmetic formulations, it is also important to eliminate inorganic salt, especially nitrate.
Isothiazolone hydrochloride salts (isothiazolone.cndot.HCl) are generated in the general prior art process for manufacturing isothiazolones. Such a process for the manufacture of a mixture of 5-chloro-2-methyl-3-isothiazolone and 2-methyl-3-isothiazolone is described in U.S. Pat. No. 3,849,430 and European Patent No. 95907. The isothiazolone.cndot.HCl is generated in the chlorination/cyclization step of this process during which either a di-(or tri)thiaodiamide or a mercaptoamide is cyclized: ##STR4##
The chlorination slurry is then filtered, then the isothiazolone.cndot.HCl cake is washed and reslurried or dissolved in the same or different solvent. In aqueous systems, a neutralizing agent such as magnesium oxide or calcium oxide is then added to yield the free base isothiazolone and a chloride salt: ##STR5##
Certain organic amines have been suggested as neutralizing agents in non-aqueous organic media in U.S. Pat. No. 4,824,957. Such organic amines produce organic amine hydrohalide salts as byproduct neutralization salts. The amount of organic amine required to neutralize the isothiazolone hydrohalide salt is difficult to determine and thus the neutralization endpoint cannot be controlled precisely. Any excess organic amine remains in the organic solvent solution of the free base isothiazolone after neutralization and contaminates the final product solution, and furthermore may also chemically react with the free base isothiazolone to produce additional byproducts. In addition, these residual amines may also act as a source of nitrosamine contaminants, if such free base isothiazolone were to be formulated to aqueous solutions stabilized with nitrate salts.
The free base isothiazolone and amine hydrochloride salt (amine.cndot.HCl) which are formed from the neutralization reaction are separated by filtering off the solid salt from the solution of free base isothiazolone. However, the amine.cndot.HCl is sparingly to appreciably soluble in the solvent, and consequently the final isothiazolone product may not be entirely salt-free.
None of the prior art processes produces an isothiazolone which is substantially pure, salt-free, and water-free.